Method of dissolving spent nuclear fuel

ABSTRACT

A NOVEL PROCESS FOR THE DISSOLUTION OF SPENT NUCLEAR REACTOR FUEL, WHILE SIMULTANEOUSLY CONTROLLING THE AMOUNT OF VOLATILE OFF-GASES EVOLVED DURING THE DISSOLUTION COMPRISES THE STEPS OF (A) PASSING SPENT NUCLEAR REACTOR FUEL, IN REGULATED AMOUNTS THROUGH A DISSOLVNET SOLUTIONS, (B) CONTINUOUSLY WITHDRAWING THE VOLATILE OFF-GASES WHICH ARE EVOLVED, (C) WITHDRAWING A PORTION OF THE SOLUTION OF SPENT NUCLEAR REACTOR FUEL WHEN A PREDETERMINED MAXIMUM CONCENTRATION OF FUEL VALUES IS REACHED IN THE SOLUTION (D) ADDING A COMPENSATING VOLUME OF DISSOLVENT SOLUTION WILE STEP (C) IS BEING CARRIED OUT, (E) CONTINUOUSLY RECIRCULATING THE REMAINING PORTION OF THE SOLUTION OF SPENT NUCLEAR REACTOR FUEL TO THE DISSOLUTION STEP, (F) DISCONTINUING STEPS (C) AND (D) WHEN A PREDETERMINED MINIMUM CONCENTRATION OF FUEL VALUES IS REACHED IN THE SOLUTION, AND (G) REPEATING STEPTS (C)-(F) AS THE PREDETERMINED MIXINUM AND MININUM FUEL VALUES ARE REPEATEDLY OBTAINED IN THE SOLUTION. IN A PREFERRED MODE OF OPERATION TWO DISSOLVER SYSTEMS ARE EMPLOYED AND FUEL TO BE DISSOLVED IS FED TO ONE DISSOLVER SYSTEM WHILE THE OTHER DISSOLVER SYSTEM IS BEING WASHED, DISCHARGE AND READIED FOR ANOTHER DISSOLUTION CYCLE.

May 28, 1974 A. L. AYERS METHOD OF DISSOLVING SPENT NUCLEAR FUEL 2'Sheets-Sheet 1 Original Filed Sept. 4, 1968 Patented .May 28, 1974United States Patent "Office 3,813,464 METHOD OF DISSOLVING SPENTNUCLEAR FUEL Arnold f. Ayers, Convent Station, NJ., assignor to AlliedChemical Corporation, New York, NY. Application Sept. 4, 1968, Ser. No.777,933, which is a continuation-in-part of abandoned application Ser.No.

1 718,175,"'Apr.' '2, 1968. Divided and this application June 28, 1971,Ser. No. 157,597

Int. Cl. C01g 43/00 US. Cl. 423-20 12 Claims ABSTRACT OF THE DISCLOSUREcontinuously withdrawing the volatile off-gases which are evolved, (c)withdrawing a portion of the solution of spent nuclear reactor fuel whena predetermined maximum concentration of fuel values is reached in thesolution, (d) adding a compensating 'volume of dissolvent solution whilestep (c) is being carried out, (e) continuously recirculating theremaining portion of the solution of spent nuclear reactor fuel to thedissolution step, (if) discontinuing steps (c) and ((1) when apredetermined minimum concentration of fuel values is reached in thesolution, and (g) repeating steps (c)(f) as the predetermined maximumand minimum fuel values are repeatedly obtained in the solution. In apreferred mode of operation two 'dissolver systems are employed and fuelto be dissolved is fed to one dissolver system while the other dissolversystem is being washed, discharged and readied for another dissolutioncycle.

BACKGROUND OF THE INVENTION Nuclear reactor fuel elements becomeunusable after a period of time due to the build-up in the fuel duringoperation of the reactor, of a variety of fission products which'serveas neutron absorbers or poisons for the reaction. Though unusable atsuch a time these fuel elements still possess a valuable quantity offissionable fuel values as well as a variety of useful by-products ofthe fission reaction. Accordingly, from an economic standpoint, it isnecessary to recover these values. Reprocessing schemes for recoveringthese values generally involve treatment of solutions in which thesevalues have been dissolved. Currently, nitric acid is by far the mostwidely used dissolvent for this purpose, although other dissolvents areknown.

During the dissolution step a variety of volatile ofigases are evolved.These off-gases include radioactive materials such as iodine, xenon andkrypton, as well as inorganic gases formed by the decomposition ofdissolvent whi'ch in the case of nitric acid, for example, may includeNO and N The uncontrolled evolution of thesevgases may overtax off-gastreatment facilities and produce dangerous situations, particularly whenlarge quantities of radioactive off-gases are released within a shortperiod of time. Government regulations control the maximum concentrationof radioactive gases that may be released at a particular site withinprescribed periods of time. Unless control is exercised over the releaseof the radioactive off-gases, the maximum tolerated concentration ofthese gases may quickly be reached, particularly where largeinstallations are involved.

A variety of means have been devised to gain control over the evolutionof the oif-gases, particularly the radio-active off-gases, of thesedissolution reactions. Controls has been exercised chemically, forexample, as described in U.S. Pat. 3,119,658 to Schulz. A variety ofmechanical methods for the control of off-gas evolution duringdissolution of nuclear reactor fuels have been attempted. None have metwith complete satisfaction, particularly when it is attempted to applysuch methods for use in large installations. For example, it has beenattempted to effect a control of off-gas evolution by adding dissolvent,in controlled amounts, to the fuel. It was found, however, that undersuch an arragement dissolution of the fuel still takes place and thateven with very small quantities of dissolvent, large quantities ofoff-gases are released within a short period of time. This isparticularly true in nitric acid systems. Moreover, when nitric acid isused as the dissolvent and added incrementally to the fuel,polymerization of plutonium values take place. The plutonium polymer canbe made to dissolve in nitric acid only with extreme difficulty, and,accordingly, the dissolution efficiency of the operation issubstantially diminished.

SUMMARY OF THE INVENTION It is an object of this invention to provide aneffective semi-continuous or continuous process for the dissolution ofspent nuclear reactor fuel elements which simultaneously affords a meansfor controlling the amounts of volatile off-gases evolved during thedissolution.

It is a particular object of the invention to provide a method asdescribed above which is particularly ap plicable to nitric aciddissolution of nuclear power reactor fuels, especially uranium oxide andplutonium oxide, and which effectively controls the evolution ofradioactive oif-gases during the dissolution, especially krypton.

Another object of the invention is to provide novel apparatus toeffectively carry out the above noted objects which is simple inconstruction and operation and which requires a relatively low capitalinvestment.

Other objects of the invention will become apparent from the followingdescription.

In accordance with the procedural aspects of the invention it has beenfound that the objects of the invention may be accomplished by carryingout the dissolution of spent nuclear fuel elements in the followingmanner. The spent nuclear fuel is passed, in a controlled manner, into adissolvent solution. The volatile off-gases which are formed arecontinuously withdrawn from the system. A portion of the solution ofdissolved spent nuclear reactor fuel is withdrawn from the system when apredetermined maximum concentration of fuel values is reached in thesolution. A compensating volume of dissolvent solution is added to thesystem while withdrawal of product solution is taking place. Theremaining portion of the solution of spent nuclear reactor fuel iscontinuously recirculated to the dissolution step. When a predeterminedminimumconcentration of fuel values in the solution is reached due todilution with the added dissolvent solution, withdrawal of productsolution and addition of dissolvent solution is discontinued and theconcentration of the fuel values in the solution is allowed to build upto thepredetermined maximum amount. This process is repeated untilcomplete dissolution of all fuel charged is accomplished. A plurality ofdissolving systems may be employed, each of which operates substantiallyas described above. Spent nuclear reactor fuel may be charged to some ofthe dissolving systems and dissolution carried out while the otherdissolving systems are being prepared for dissolution cycles. If asingle dissolving system is employed it may be operated in a continuousfashion until it can no longer tolerate any additional charge of fuel.At this point dissolution is stopped, the leached hulls are dis-chargedfrom the system and the system is readied for another dissolution cycle.This mode of operation will be termed semi-continuous herein.Preferably, two dissolving systems are employed, with dissolution takingplace in one system with the other system being prepared fordissolution. With this arrangement dissolution can be carried outaround-the-clock and this mode of operation will be termed continuousherein.

In accordance with the structural aspects of the invention it has beenfound that the objects of the invention may be accomplished by carryingout the procedures described above in apparatus which comprises aplurality of dissolving units equipped with means for feeding spentnuclear reactor fuel, in a controlled manner, to a dissolving unit or toa particular member or members of the dissolving units, each of whichdissolving units comprises: (a) a fluid retaining main housing memberopen at one end and having a fluid inlet means at the opposite end, (b)a fluid retaining recycle housing member open at one end which open endis connected to the open end of the fluid retaining main housing member,the opposite end of the fluid retaining recycle housing member beingconnected to the opposite end of the fluid retaining main housingmember, (c) means for circulating a dissolvent through the fluidretaining main housing member to the fluid retaining recycle housingmember and back to the fluid retaining main housing member, and (d)means for withdrawing a portion of the dissolved fuel solution from thefluid retaining recycle housing member. In a preferred mode, a removableperforated basket-like member is situated within the fluid retainingmain housing member of each dissolving unit and is adapted to supportthe divided spent nuclear reactor fuel for contact with the dissolvent.These removable basketlike members facilitate removal of the leachedhulls, washing of the unit and preparation of the dissolving unit forthe next dissolving cycle. In a preferred embodiment, two dissolvingunits are employed and a common reactor fuel feeding means is employedfor both dissolving units.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevation, in partialsection, of the preferred embodiment of the apparatus of this inventionand shows a twin dissolving unit with one unit cut away, together withthe preferred form of diverting means for the distribution of fuelmaterial to the respective dissolving units. The cutaway dissolving unitis identical to the one shown and has been omitted to simplify thedrawing.

FIG. 2 is a cut-away, in partial section, showing an alternate form ofdiverting means to one dissolving unit of a twin dissolving unit system.As in FIG. 1, one complete dissolving unit has been cut away.Additionally, only a portion of the remaining dissolving unit is shown,which demonstrates an alternate mode of connecting the diverting meansto the dissolving unit.

DETAILED DESCRIPTION OF THE INVENTION AND OF THE PREFERRED EMBODIMENTSThe invention process and apparatus are applicable to the dissolution ofa variety of nuclear fuel elements with a variety of dissolventsolutions. Spent nuclear reactor fuels derived from light watermoderated and cooled reactors, gas cooled reactors, fast converters andthermobreeders are illustrative type fuels which may be employed. Thefuels may be power type fuels from power stations or non-power typefuels from test and research reactors. Illustrative candidate power fuelmaterials include the common metallic oxide type nuclear reactor fuelssuch as uranium oxide (U0 plutonium oxide (PuO and mixed uraniumoxide/plutonium oxide or uranium oxide/thorium oxide (ThO fuels.Carbides or alloys of uranium, plutonium and thorium or of other fissileand fertile materials are also suitable. Illustrative suitable fuelsfrom test and research reactors include plates of U0 uranium metal,uranium carbide, alloys of uranium and aluminum, alloys of zirconium anduranium, uranium nitrides, uranium hydrides and similar compounds of Pu.Other fuel elements suitable for use in accordance with the inventioninclude uranium or plutonium type elements in combination with niobiumand/ or graphite, CaO, Si compounds and other inert metals. Still otherfuel materials suitable for use in accordance with the invention willreadily occur to those skilled in the art.

The type of cladding associated with the element of the particular fuelwhich is to be treated is not critical. It may include, for example,zirconium, alloys of zirconium, stainless steel, aluminum and othermetals. It is preferred, however, that the cladding associated with thefuel to be dissolved be inert to the dissolvent to be employed. Thisfacilitates separation of the desired values from the dissolved fuelsolution.

For reasons indicated above, the nature of the dissolvent is notcritical but nitric acid is preferred. With certain types of fuels,however, it may be desirable to incorporate additional ingredients withthe dissolvent to achieve special purposes. For example, in the case ofmixed UO /ThO fuel which is not readily dissolved by concentrated HNOalone, it is desirable to add NaF to the HNO dissolvent since fluorideion in combination with HNO promotes dissolution of this type of fuel.Uranium-molybdenum alloy type fuels generally require the addition of asource of ferric ion, e.g. Fe(NO in order to promote dissolution. HF isgenerally added to HNO dissolvent when zirconium-uranium alloy typefuels are treated. However, if the ratio of uranium to zirconium is low,HF may be the dissolvent or small quantities of HNO may be added to thedissolvent. Mercuric nitrate catalyzes the nitric acid dissolution ofaluminum-uranium alloy type fuels.

In accordance with standard procedures, soluble neutron poisons such asboric acid, cadmium nitrate, gadolinium nitrate, or any other materialwhich has a high neutron absorption cross-section, may be added to thedissolvent to allow the amount of fissile material in the dissolver toexceed the nominal critical quantity without causing the system tobecome critical.

For the reasons given above, it is to be stressed that neither theprocedural nor structural aspects of this invention are to be limited bythe type of fuels dissolved, the type of dissolvent used, or by anyadditive which may be added to the system to achieve special purposes.It should be kept in mind, however, that chemicals added to the fuel ordissolvent constitute contamination which must ultimately be removed,thereby complicating the recovery system.

The fuel elements to be treated may vary substantially in size as usedin the nuclear reactor. Non-power type fuel elements may be used inlengths as small as 18 inches and below whereas power type fuel elementsare often in the range of about |l0-14 feet in length. Although,virtually any size fuel element can be treated in accordance with thisinvention providing appropriately sized equipment is employed, it isusually preferred to divide the fuel elements into smaller pieces tofacilitate handling and to expedite the dissolution. This may beaccomplished by any conventional means such as shearing units which cutthe fuel elements into small pieces. The divided fuel elements are thentreated in accordance with the invention. Some types of fuel elements,such as those comprising a perforated graphite matrix filled With fuelvalues, could be crushed to form a more suitably sized fuel feed. Theinvention will be particularly described with reference to a shearingstep for the fuel elements and more particularly to the use of a singleshear and twin dissolver units for continuous operation. From the abovediscussion it will be clear that this is illustrative only and is not tobe taken as a limitation on the invention. Separate shears could beemployed for each dissolving unit or none at all for that matter.Semicontinuous operation could be carried out in a single dissolverunit.

With reference to FIG. 1, 1 is a diverting valve which receives choppedor sheared fuel elements from a shear (not shown) through stem 2 anddiverts the chopped fuel elements through leg 3 or leg 4 of thediverting means. Legs 3 and 4 transport the chopped fuel elements totwin dissolving units. Only the dissolving unit which is connected toleg 3 is shown and is designated 5.

The chopping or shearing unit is conventional and is not a part of thisinvention. It may be operated manually or b'y automatic hydraulic powerand serves to chop or shear the fuel elements into preselected lengthssuitably in the range of about 1-3 inches. The length of the choppedpieces of fuel element is not critical but dissolution rates will varywith the length of the pieces depending on the extent of burn-up of thefuel. The lesser the burn-up, the lower will be the dissolution rate andsize of the cuts of the fuel element can be reduced accordingly toincrease dissolution rates. The optimum size of the cut for a particularfuel and a specified burn-up can be readily ascertained by routineexperimentation.

The particular structure of the diverting means is not critical. Theonly critical requirement for the diverting means is that it provide ameans for regulating the flow of chopped fuel to the respectivedissolving units. This may be accomplished by providing flapper valveswhich can be used to permit or prevent flow of chopped fuel to thedissolving units. The rate of addition of chopped fuel to a dissolvingunit in which dissolution is taking place is regulated to maintaincontrol of the dissolution to avoid peaking or an undesirably largeevolution of offgases within a short period of time. In a preferredembodiment, when two dissolving units are employed, the diverting meansis designed so that when flow of chopped fuel is taking place to onedissolving unit, flow of chopped fuel cannot take place to the otherdissolving unit and vice-versa. In a still preferred embodiment, meansis provided to close access to the transport piping leading to theshearing unit so that after a charge of chopped fuel is fed to adissolving unit, evolved off-gases from that unit are prevented fromescaping through the transport piping to the shearing unit. Thepreferred structure of the diverting means is shown in FIG. 1 and analternate structure is shown in FIG. 2. The preferred structure of FIG.1 shows a bucket-like member 6, which is situated in coaxial alignmentbelow stem 2 so that chopped fuel element material which drops throughstem 2 falls into bucket-like member 6. The impact of the fall ofchopped fuel is thus buffered by bucket-like member 6. Flapper unit 7controls the feed of chopped fuel from stem 2 to bucket-like member 6.Bucket-like member 6 is mounted on a pivot point 8 which permits it tobe tilted to either side thereby dumping the contents of the bucket-likemember either into leg 3 or leg 4. After dumping, bucket-like member 6is returned to an upright posi tion and is ready to receive another cutof fuel. The instrument controls for tilting the bucket-like member arenot shown. Such are within the skill of the art. Flapper units 9 and 10control access from chamber 11 containing bucket-like member 6 to legs 3and 4, respec tively. Chopped fuel material which is permitted access toleg 3 falls into perforated basket-like member 12 which is positionedinside of fluid retaining main housing member 13 of dissolving unit 5.Basket-like member 12 contains a lip 15 which rests on shoulder 16 ofdissolving unit 5 to support this member. After a controlled amount ofchopped fuel has been fed to dissolver unit 5, flapper unit 9 is closedto prevent off-gases from the dissolving unit to escape through thediverting means. Since a small amount of evolved off-gases willinvariably escape into chamber 11 of the diverting means 1 upon initialcontact of the chopped fuel with the dissolvent in dissolving unit 5before there is opportunity to close flapper unit 9; flapper unit 7 isclosed after charging bucket-like member 6 with a cut of fuel beforedumping takes place into leg 3. Chamber 11 may be purged by any suitablemeans such as a steam purge (not shown in the drawing).

Dissolvent is charged or added to the system through pipe 14. Atstart-up, dissolvent is charged to the system in an amount to provide asufiicient liquid level in the dissolving unit and perforatedbasket-like member 12 to effectively contact the chopped fuel piecesdropped in the basket-like member.

Conditions maintained in the dissolving unit will vary depending uponthe type of fuel, the type of dissolvent and the type of additivepresent. In the case of uranium oxide fuel and nitric acid dissolvent,for example, dissolution temperatures will range between about 50 120C., and will preferably be at least about C. The particular dissolutionconditions in the situation involved, such as temperature andconcentration of dissolvent employed, are strictly conventional and donot form a part of this invention. Generally, when nitric acid is usedas dissolvent, a 6-13 M solution may be employed as charge to thedissolver and more usually as a 79 M solution. During the dissolutioncycle, at equilibrium, HNO dis solvent will gradually assume aconcentration of 1-7 M or more usually a concentration of 2-3 M.

Temperature conditions in the main housing member 13 and basket-likemember 12 of the apparatus may be maintained by providing any suitableheat exchange means around a portion of the main housing member, such aswater jacket 17, equipped with the usual inlet and outlet means 18 and19, as shown in the drawing. This function may be accomplished by othermethods, however, for example, by heating coils or wires wrapped arounda suitable length of the main housing member. An alternate way ofcontrolling temperature is to heat or cool only the recycle housingmember thereby eliminating the need for main housing member Water jacket17. If desired, heat exchange means may be provided along portions ofboth the main housing member 13 and recycle housing member 23.

The optimum amounts of neutron poisons to be incorporated with thedissolvent, if employed, may be 'readily determined by one of ordinaryskill in the art.

The optimum amounts will depend on reactor design and criticalities ofthe fissionable material present and are within conventional knowledgeand do not form a part of this invention. Generally, if boric acid is tobe used as a poison together with nitric acid, for example, about l-20grams/liter of boric acid/nitric acid may be effectively employed.

The solution of dissolvent and dissolved fuel elements, together withvolatile off-gases which are produced, are forced upwardly through theapparatus by virtue of the pressure differential which is built up inthe system. A removable cover or lid portion 21 is provided to seal offthe fluid retaining main housing member 13 from gas evolution duringdissolution. The dissolved solution and off-gas mixture then passesthrough upper connecting sec tion 22 and from there into fluid retainingrecycle member 23. Optionally, but preferably, a screen (not shown inthe drawing) is provided at the entrance to the upper connecting sectionto screen out particles of fuel or particles of leached hulls thereofwhich otherwise may be carried over into the recycle housing member 23,thereby presenting a diflicult problem of removal. The offgases enteringrecycle housing member 23 are vented off through the open top of therecycle housing member at 24 and are further processed in a conventionalmanner such as by scrubbing, iodine removal, filtering and eventualdischarge to the atmosphere. The dissolved fuel solution drops down intothe body portion of recycle housing member 23 and is available forrecirculation or take-off as needs dictate, as will be explained in moredetail hereinafter.

In the preferred mode of operation, at start-up of the dissolver,chopped fuel is fed to the dissolving unit which has already beencharged with the desired amount of dissolvent solution. A pressuredifferential is created in the system by maintaining a portion ofrecycle housing member 23 at a temperature lower than the temperaturemaintained in main housing member 5. This may be accomplished by anyconventional heat exchange means such as cooling jacket 27 equipped withthe usual inlet and outlet means 28 and 29, respectively. Equivalentheat exchange means may readily be devised. Gas evolved duringdissolution or inert gas added to the main housing member 13 throughinlet 14 may also be used to create a pressure differential forrecirculation. This pressure differential provides a circulating meansfor the solution in the dissolver unit. After a predetermined amount ofthe sought-for fuel values has gone into solution, as may be ascertainedby continuous testing with routine equipment, a portion of the productsolution is withdrawn from recycle housing member 23 through outlet 25.The concentration of the product solution withdrawn is selectedaccording to subsequent needs of the reprocessing scheme which isdesigned to recover fuel values from this solution, for example, byconventional solvent extraction methods which are not a part of thisinvention. Ordinarily, the product solution is withdrawn at aconcentration which is somewhat greater than that which is required forsubsequent treatment. It may be readily diluted for use as required. Theremaining portion of the product solution which is not withdrawn, isrecirculated as described above to the dissolution step. The recyclingor recirculating is a critical part of the invention process. Therecycling step serves to avoid the danger of reducing the acidity in thedissolver to the point where undesirable polymerization of plutoniumvalues takes place. The recycling step also serves to agitate thedissolvent solution in the dissolver, thereby simplifying the apparatusdesign.

In order to compensate for the volume of product solution withdrawnthrough outlet 25, a like volume of fresh dissolvent solution is fedinto main housing member 13 through inlet 14. When this is done, thefuel values in the dissolvent solution will be diluted and the fuelvalue concentration will go down. When the concentration of the fuelvalues reaches a preselected minimum value, depending on considerationsrelating to concentrations of fuel values desired for subsequentreprocessing, as described above, product solution withdrawn is stoppedas well as compensating dissolvent addition, and dissolution of the fuelis allowed to proceed. Eventually, the concentration of the fuel willreach its preselected maximum value, whereupon product solution is againwithdrawn and additional dissolvent is provided. This cycle is repeateduntil all the fuel values have been extracted from all the chopped spentfuel material which can be added to the dissolving unit.

The optimum proportion of product solution to be withdrawn to the amountof product solution recycled will depend on a variety of factors such astype of fuel dissolved, the extent of enrichment of the fuel, the extentof burn-up of the fuel, the type of dissolvent employed and the desiredconcentrations of dissolved fuel values in the product solution forsubsequent processing, as described above. The best proportion to use ina particular case may readily be determined by anyone skilled in the artbased upon these and other obvious considerations with routineexperimentation.

After the dissolution cycle is complete or substantiall y complete indissolving unit 5, chopped fuel material is fed into bucket-like member6 of diverting means 1 and is diverted through leg 4 to the twindissolving unit connected thereto. Dissolving unit 5 may then beprepared for an additional dissolution cycle. The leached hulls inbasket-like member 12 may be washed in situ with fresh acid and water.Access to basket-like member 12 is obtained by opening removable coveror lid member 21. The basket-like member containing the washed leachedhulls may be monitored to determine if significant residual fuel 'valuesare contained therein in which case the hulls are retreated to recoverthe same, or if no significant residual fuel values are present, thebasketlike member 12 is lifted out of main housing member 13 with theaid of handle 30 and the leached hulls contained therein treated anddisposed of in a conventional manner. At this point, dissolver unit 5 isready for load ing with an empty basket-like member and for anadditional dissolving cycle. When a dissolution cycle is completed inthe twin dissolver unit, that unit can be prepared and recharged for anadditional cycle while dissolving unit 5 can be operated. In this manneran effective continuous around-the-clock dissolution process isachieved.

Material of construction of the apparatus should be of a metal which isinert to the action of the dissolvent employed. Stainless steel is asuitable construction material when nitric acid is employed.

FIG. 2 shows an alternate form of diverting means and an alternate meansof tying the same into the dissolving units. Referring to FIG. 2, 40 isthe alternate diverting means, 41 is the stem and 42 and 43 are thelegs, which parts are analogous to those in the diverting means shown inFIG. 1. Flapper unit 44 controls the flow of chopped fuel elements fromthe shear through stem 41. Another flapper unit 45 in the stern, whenclosed, serves as a buffer for the fall of chopped fuel materialanalogous to the buffer provided by the bucket-like member 6' in thediverting means of FIG. 1. After a cut of fuel is dropped onto flapperunit 45, flapper unit 44 may be closed. Diverting flapper unit 46 canthen be positioned to one side or the other to divert the chopped fuel,when dropped, into either leg 42 or leg 43. When flapper unit 45 isopened, permitting the chopped fuel to drop into one of the legs 42 or43 of the diverting means, closed flapper unit 44 serves to block theescape of evolving gas from the dissolving unit which is then incommunication with stem 41, into the stern. With the position ofdiverting flapper unit 46 shown in the drawing, the dropped fuelmaterial will be fed through leg 42 directly into basket-like member 47of main housing member 48 of dissolving unit 49. As may be seen in FIG.2, basket-like member 47 has an opening 58 in its side which directlyconnects to leg 42 of the diverting means. Otherwise, dissolving unit 49is wholly analogous to the dissolving unit 5 of FIG. 1. The parts inFIG. 2 which are analogous to similar parts shown in FIG. 1 are lip 50,shoulder 51, water jacket 59, water jacket outlet 52, open top 53 ofmain housing member 54, removable top member 55, upper connecting member56 and handle 57.

In a typical operation carried out in apparatus as described in FIG. I,spent U0 fuel elements, clad with Zircaloy, from a typical light-watercooled and moderated reactor, are sheared into 2" pieces and a cutranging between about 25-50 lbs. is dropped into bucket member 6 whichhas a capacity of about lbs. Stem member 2 and leg member 3 havediameters of about 16". Dissolving unit 5 is charged with a nitric aciddissolvent solution containing boric acid to a level whch reaches aboutthe height of basket member 12. Basket member 12 has a diameter of about25" and is 5 high. Flapper units 7 and 10 are closed, flapper unit 9 isopened and bucket member 6 is tilted to discharge its contents ofchopped fuel into basket member 12. Upon contacting the acid solution,the soluble components in the fuel begin to dissolve and gaseousoff-products are released. When the uranium concentration in thesolution reaches about 275-300 gms./liter, product solution is withdrawnthrough outlet 25. At the same time an equivalent volume of nitric aciddissolvent solution containing boric acid is fed into the dissolverthrough inlet 14. When the concentration of uranium in the solutionfalls to about 250 gms./1iter, withdrawal of product is stopped as wellas addition of nitric acid and boric acid solution. Periodically,additional cuts of chopped fuel are dropped into basket-like member 12.These cuts are added in an amount and at a rate such that the release ofgaseous products is spread more or less evenly over a period of about -9hours. This may be accomplished by feeding in cuts containing 25-50 lbs.of chopped fuel material with about 5-10 minutes allowed in betweencuts. After all these cuts have been added, dissolution is continued fora period of about 3-4 hours. During this final dissolution period,particularly in the latter stages, chopped fuel material may be chargedto the twin dissolver unit and dissolution commenced therein. After thefinal dissolution period, dissolution unit 5 may be prepared for anothercycle. This is accomplished by draining the dissolution unit andcharging the unit with fresh acid solution to dissolve any residual fuelpresent. This acid solution is then drained and the dissolution unit isfilled with water to rinse out residual acid. The water is then drainedfrom the unit and the unit is purged with steam. Lid member 21 is thenopened and the basket containing leached hulls is removed. The basket isthen monitored to detect the presence of any residual fissile materialin which case it is returned to a dissolving unit for further treatmentor, if no significant amount of residual fissile material is present,the leached hulls are removed, treated and disposed of in a conventionalmanner.

It will be apparent to anyone skilled in the art that a variety ofmodifications and variations may be made to the novel process andapparatus described herein without departing from the scope and spiritof the invention. For example, the apparatus may be equipped withdetectors to provide a warning when criticality limits are approached.Means can be provided to admit a gas to the bottom of the dissolverunits to provide auxiliary mixing if it becomes desirable to do so.Screens and filters may be provided to control the passage of solidparticles throughout the system. Steam jets may be provided to emptyliquids from the dissolving units. Steam or other gases may be used forpurging the diverting means unit. The basket-like members may bepartially perforated in lieu of being fully perforated to influencedissolution rates. Recovery and recirculating equipment for useddissolvent may be associated with and tied into the subject apparatus. Amultitude of instrumentation lines and associated equipment may beprovided to control the various flapper units, to measure theconcentrations of fuel values in the dissolvent product solution, tomaintain liquid levels and for other purposes. Many obviousmodifications and refinements will readily occur to those skilled in theart. Accordingly, the invention is to be limited only by a reasonableinterpretation of the scope of the appended claims.

I claim:

1. A semi-continuous process for the dissolution of spent nuclearreactor fuel, while simultaneously controlling the amount of volatileoff-gases evolved during the dissolution which comprises:

(a) passing spent nuclear fuel, in regulated amounts, into a dissolventsolution to effectuate dissolution of the soluble components in thefuel,

(b) continuously withdrawing the volatile off-gases which are evolved,

(c) withdrawing a portion of the dissolved solution of spent nuclearreactor fuel when a predetermined maximum concentration of fuel valuesis reached in the solution,

(d) adding a compensating volume of dissolvent solution while step (c)is being carried out,

(e) continuously recirculating the remaining portion of the dissolvedsolution of spent nuclear reactor fuel to the dissolution step,

(f) discontinuing steps (c) and (d) ,when a predetermined minimumconcentration of fuel values is reached in the solution, and

(g) repeating steps (c)-(f) as the predetermined maximum and minimumfuel values are repeatedly obtained in the solution.

2. A process for dissolving spent nuclear reactor fuel as described inclaim 1 in which the spent nuclear fuel which is processed is derivedfrom a thermal reactor.

3. A process for dissolving spent nuclear reactor fuel as described inclaim 2 in which the dissolvent is HNO 4. A process for dissolving spentnuclear reactor fuel as described in claim 3 in which the fuel is amember selected from the group consisting of U0 and PuOg and mixed fuelsselected from the group consisting of UOg/ Pu0 and UO /ThO 5. A processfor dissolving spent nuclear reactor fuel as described in claim 4 inwhich the fuel is U0 6. A process for dissolving spent nuclear reactorfuel according to claim 1 in which quantities of divided spent nuclearfuel are charged in a plurality of dissolving systems each of whichoperates substantially as described in claim 10.

7. A process for dissolving spent nuclear reactor fuel as described inclaim 6 in which there are two dissolving systems.

8. A process for dissolving spent nuclear reactor fuel as described inclaim 7 in which process dissolution is caused to take place in onedissolving system while the other dissolving system is being preparedfor a dissolving cycle.

9. A process for dissolving spent nuclear reactor fuel as described inclaim 8 in which the spent nuclear reactor fuel is derived from athermal reactor.

10. A process for dissolving spent nuclear reactor fuel as described inclaim 9 in which the dissolvent is HNO 11. A process for dissolvingspent nuclear reactor fuel as described in claim 10 in which the fuel isa member selected fromthe group consisting of U0 and Pu0 and mixed fuelsselected from the group consisting of UO PuO and UO /ThO 12. A processfor dissolving spent nuclear reactor fuel as described in claim 11 inwhich the fuel is U0 References Cited UNITED STATES PATENTS 3,565,587 2/1971 Graf 23--342 X 3,374,068 3/1968 Erlandson et al. 23341 X 3,222,12412/1965 Anderson et a1. 23-324 X OTHER REFERENCES Newby et al.: AnImproved Aqueous Process for Zr Allar Nuclear Fuels, report No.-IDO-14674, 1966, pp. 14.

Stoller et al.: Reactor Handbook, 2nd ed., vol. II, 1961, pp. 68-70, and94.

Flagg: Chemical Processing of Reactor Fuels, 1961, p. 109.

CARL D. QUARFORTH, Primary Examiner R. L. TATE, Assistant Examiner U.S.Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECT-MN Patent No. U Q 31-3 146,4 Dated May 28 197 Inventor(s) ARNOLD L. AYERS It is certifiedthat error appears in the above-identified patent and that said. LettersPatent are hereby corrected as shown below:

Col. 2, line 6: "trols" should read trol Col. 2, line 16 "arragement"should read arrangement (301.10, line- 30 (claim 6) "in claim 10" shouldread I in claim 1 Signed ad sealed this 26th day of November 19745(SEAL) Attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner. ofPatents USCOMM-DC 60376-P69 w uls. GOVERNMENT PRINTING OFFIC'E: I9690-366-334 FORM PO-105O (10-69) UNITED STATES PATENT OFFICE CERTIFICATEOF CORR'ECTEQN Patent No. U 3 3 l3 14614 Dated May 28 197 Inventor(s)ARNOLD L. AYERS It is certified that error appears in theabove-identified patent and that said. Letters Patent are herebycorrected as shown below:

Col. 2, line 6 "trols" should read tro-l Col. 2, line 16 "arragement"should read arr-angement Co1.'l0, line 3.0 (claim 6) "in claim 10"should read in claim 1 Signed and sealed this 26th day of November 1974:

(SEAL) Attestz McCOY M. GIBSON JR. C. MARSHALL DANN Attesting QfficerCommissioner. of Patents USCOMM-DC 60376-P69 3 FORM PO-105O (10-69) aU5. GOVERNMENT PRlNTING oFncr: was o-sss-su

